Assessing the Benefits of Surface Tension Transfer®

1Journal of Industrial Technology Volume 19, Number 4 August 2003 to October 2003 www.nait.org

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The Official Electronic Publication of the National Association of Industrial Technology www.nait.org 2003

Assessing the Benefits of SurfaceTension Transfer Welding to Industry

By Mr. Bruce D. DeRuntz

Volume 19, Number 4 - August 2003 to October 2003

Peer-Refereed Article

ManufacturingMaterials & Processes

MetalsProduction

Welding


Assessing the Benefits ofSurface Tension TransferWelding to IndustryBy Mr. Bruce D. DeRuntz

Bruce DeRuntz is an Assistant Professor in the De-partment of Technology at Southern Illinois Uni-versity Carbondale. Bruce develops and teachescourses in Quality Control, Industrial Metrology,Manufacturing Policy, Manufacturing Processes,and First-line Supervision. He is a past recipient ofhis departments Outstanding Teacher award andcurrently serves as President Elect of NAITs In-dustry Division. Bruces industrial experience in-cludes 10 years with defense and automotive com-panies designing and implementing quality sys-tems, supervision and administration, projectmanagement, and supplier development. His con-sulting experience includes three years with theManufacturing Extension Partnership. He is a Cer-tified Quality Engineer, Certified Senior IndustrialTechnologist, and ISO 9000 Auditor.

The Lincoln Electric Company isthe first and only welding company tohold a patent on a revolutionary newwelding process called Surface TensionTransfer (STT). Originally pat-ented in 1988, it wasnt until 1994 thatthe first commercial unit was producedand sold. Today this welding process isbeing used by every major pipecontractor in the world, yet it is stillrelatively unknown in many manufac-turing sectors. To test if weldingeducators recognized this weldingprocess, educators at a NationalAssociation of Industrial Technology(NAIT) conference were asked if theyhad ever heard of STT. A unanimousresponse indicated that they had neverheard of this new process (DeRuntz,2001). The audience expressed astrong interest in learning more aboutthis new technology and encouragedthe presenter/author to publish a paperthat would contribute to the body ofknowledge for Industrial Technologists.Therefore, the purpose of this paper isto enlighten Industrial Technologyprofessionals on one of the newesthigh-tech welding processes, STT, andsuggest the future implications formanufacturing adopting this technol-ogy. This paper explains how theprocess works, examines its advantagesand disadvantages, and provides casestudies of its successful implementa-tion in the world of manufacturing.While this paper is not intended toendorse any manufacturer or itsproducts, the technology explainedwithin is patent protected by TheLincoln Electric Company.

Process DescriptionSTT is a new approach to what has

been known as the Short Arc transfermode or Short Arc welding. To bestexplain this process, it is first necessary

to review the characteristics that aresought after in the Short Arc processes.The popular solid wire, Short Arcwelding process is primarily selectedfor applications in manufacturing thatrequire medium to low heat input. Theprocess will work in all positions andonly requires average operator skill. Inthis process, the operator sets the wirefeed speed and average voltage, basedupon the heat required for the particu-lar application. This takes into ac-count, but is not limited to, factors suchas material size and type, joint configu-ration, electrode size and type, travelspeed, and arc shielding gas. TheShort Arc process is characterized byits undesirable explosion of moltenmetal known as spatter. Spatter occurswhen the electrode makes contact withthe base metal (shorting out thecircuit), then a high current, known asPinch Current is applied to blow orseparate this short. The molten dropcontacting both the electrode and workacts like a fuse and blows, depositingsome of itself into the weld path andsurrounding fixtures, while castingother parts into the air. This processrepeats itself about a hundred times persecond as the machine tries to maintainthe set voltage. To gain control of thisvolatile welding process and producehigher quality welds, the power sourceneeds to be better controlled.

The TIG welding process preciselycontrols the current through a highlyskilled operator that uses a foot pedalor hand control to continuously adjustthe amount of current that he deemsnecessary during every second of thewelding process. The characterizationof this process is a much slowerdeposition rate of filler metal, butresults in higher quality welds withoutspatter. The STT process uses sophisti-cated electronic technology to combine


the best characteristics of the Short Arcand TIG processes. The STT processcould be called an intelligent TIGwelding process for Short Arc welding.

The welding scientists whoinvented this process have customdesigned the weld current (waveform)to be modified hundreds of times persecond to transfer each droplet ofmolten metal when the electrode isshorted, such that there is no volatileexplosions, and thus eliminatingspatter. According to Dodson (1999)the key to STT technology is its abilityto control the current independent ofwire feed speed. This means that moreor less current can be applied withoutadding more wire. The SurfaceTension Transfer process was namedafter the way this technology monitorsand controls the surface tension of theweld droplet as it adheres to the weldpuddle. It does this through a high-speed inverter that precisely adjusts theoutput current waveform during theentire shorting cycle. This unique highfrequency inverter technology is knownas Waveform Control Technology

The Waveform Control technologyhas the capability of programming thepower supply for unique waveforms tooptimize the arc characteristics for aspecific application. Factors such asthe type of joint, material and thick-ness, rate of travel, electrode size andtype, as well as the specific arc shield-ing gas are all considered. Once theprogram(s) are entered into the powersupply, the optimal arc for that applica-tion is obtained, making this technol-ogy very versatile for a variety ofapplications and base materials.

According to Stava (2000), theSTT process operates neither in theconstant current nor constant voltagemode, rather it is a high-frequency(wide-bandwidth), current-controlledmachine wherein the power to the arcis based on the instantaneous arcrequirements, not on an average DCvoltage. In principle, it is a powersource that is capable of delivering andchanging the electrode current in theorder of microseconds. Furthermore, itis designed for semiautomatic applica-tions, where rate of travel, speed, andelectrode extension lengths will vary.

The applications are identical to thoseassociated with the standard short-circuiting processes. Various shieldinggases, including 100% carbon dioxideand blends of carbon dioxide and argonfor mild steel, as well as gas blendswith helium for stainless steel, may beused with this new power source. Toillustrate how this waveform technol-ogy works, figures 1 and 2 contrast thewelding processes of STT and ShortArc.

The following six steps (see figure1) illustrate the distinct steps that occurin the STT process (Stava, 2001):

1. Background current (T0 - T1):This is the current level of thearc prior to shorting to the weldpool. It is a steady-state currentlevel, between 50 and 100amperes. The electrode ap-proaches the work piece.

2. Ball time (T1 - T2): Just beforethe electrode is about to com-plete the short (at the back-ground current), the voltagesensing clip reads a decrease involtage and the machine dropsthe amperage. (In conventionalshort circuit welding, the shortcircuit would occur and amper-age would rise dramatically).The background current isfurther reduced to 10 amperesfor approximately 0.75 millisec-onds. This time interval isreferred to as the ball time.

3. Pinch mode (T2 - T3): Wire isstill being fed; therefore fusion isoccurring between the electrodewith the work piece. In order totransfer the molten drop, amper-age must be increased. A highcurrent is applied to the shortedelectrode in a controlled manner.This accelerates the transfer ofmolten metal from the electrodeto the weld pool by applyingelectronic pinch forces. (Notethat the electrode-to-workvoltage is not zero during thisperiod. This is due to the highresistivity of iron at its meltingpoint of 1550 C / 2822 F.) AtT-3 the wire begins to neckdown or melt from the outsidein.

4. The dv/dt calculation (T2 - T3):This calculation indicates themoment before the wire com-pletely detaches. It is the firstderivative calculation of the rateof change of the shorted elec-trode voltage vs. time. Whenthis calculation indicates that aspecific dv/dt value has beenattained, indicating that fuseseparation is about to occur, thecurrent is reduced again to 50amperes in a few microseconds.This is to prevent a violentseparation and explosion thatwould create spatter. (Note: thisevent occurs before the shortedelectrode separates). T4 indi-cates the separation has oc-curred, but at a low current.

5. Plasma boost (T5 - T6): Amper-age is again increased and acontrolled uniform separationtakes place and creates the weldbead with little spatter. It is atthis period of high arc currentthat the electrode is quicklymelted back. (The geometry ofthe melted electrode at this pointis very irregular).

6. Plasma (T6 - T7): This is theperiod of the cycle where the arccurrent is reduced from plasmaboost to the background currentlevel. In this tail-out period,the current goes from this higherlevel down to its initial back-ground level. The cycle thenrepeats itself, with the timerequired for one waveformtaking between 25-35 millisec-onds.

Figure 1 depicts the process duringone STT waveform. The figureillustrates amperage over time, with thetime being in milliseconds.

Comparison of TraditionalGMAW to STT GMAW

A comparison with conventionalGMAW further improves the under-standing of the STT process. In theconventional GMAW short-circuitingprocess, a high level of spatter andsmoke results from electrode separa-tion occurring at a high pinch current


compared to STT (see figure 2). In thismode, the magnitude of the current isrelatively high at the moment thedroplet separates from the wire,causing the fuse to explode andgenerating a high level of spatter(Dodson, 1999).

Figure 2 illustrates the six distinctsteps that occur in a conventionalGMAW process (Stava, 2001):

1. The electrode approaches thework piece with the amps andvolts maintaining steady levels.

2. As the electrode shorts, thevoltage drops dramatically andthe amperes begin to rise. Thenext two steps differentiateconventional GMAW from STTbecause of STTs precise controlof the waveform.

3. The electrode has come incontact (shorted) with the workpiece and is depositing the fillermetal. At this point, the voltageis approximately zero, and theamperes have increased im-mensely.

4. The increase in amperes causesthe filler metal to separate fromthe rest of the electrode in aviolent and unpredictablemanner, producing greaterspatter and smoke than STT.

5. After the separation between theweld deposit and the weldingwire, the voltage and amperesdecrease back to their presetlevels.

6. Repeats the process as in stepone.

The comparison of these twoprocesses is especially dramatic in pipewelding. The constant voltage GMAWprocess normally used for pipe weldingdoes not control the current directly;instead it controls the average voltage.This can cause the weld puddletemperature or fluidity to be too high,and the internal bead may shrink backinto the root, a reaction known as suckback. Also, when using conventionalShort Arc GMAW, the operator mustconcentrate the arc on the lip or leadingedge of the puddle to ensure properpenetration and fusion. If the arc is too

far back on the puddle, penetration willbe incomplete.

The STT process also makes itpossible to complete open root weldsthree or four times faster than GTAW,with low heat input and no lack offusion (Stava, 2001). With STT technol-ogy, the heat affected zone is minimized.Moreover, while most conventionalwelding processes can have heat inputsas high as 25,000 to 30,000 joules per

inch, the STTs heat input is typically7,000 joules per inch, which ultimatelyleads to reduced distortion. For pipewelding, the process also makes it easierto perform open gap root pass welding,with better back beads and edge fusion.It is easier to operate than other pro-cesses, yet produces consistent, X-rayquality welds.

Figure 1. Surface Tension Transfer Process.

Figure 2. Conventional Short Circuiting Process.

Figures 1&2 a provided courtesy of The Lincoln Electric Co., Cleveland, OH, USA


Table 1 provides a completecomparison of the GWAW weldingprocess to STT.

Advantages, Benefits andLimitations

The description of the processesand the technical comparison of STT toconventional Short Arc waveformsprovide an important understanding ofthe similarities and differences betweenthe two methods. To understand theworkplace attributes, STTs advan-tages, benefits, and limitations will becategorized into weld quality, operationcosts, and operation conditions.

Improved weld quality can beachieved in all position weldingthrough better fusion in poor fit upareas, lower heat input and less oxi-dization. Better fusion is possiblethrough the precise control of amper-age throughout the entire weld cycle.The precise control of the currenteliminates the volatile explosion ofmolten metal when the arc shorts,therefore depositing more of theconsumable electrode and concentrat-ing the arc on the base metal. Theability to concentrate the arc also aidsin the elimination of cold lapping onopen root joints for pipe and pressurevessels. The lower heat input providesthe advantage of less material distor-tion and burn-through by providingonly the required amount of heat toproduce the weld, even in sensitivematerial like stainless steel. Thisprecise control of heat means that eventhin gauge galvanized sheet metal canbe welded without burning off thegalvanized plating on the back side ofthe metal.

A reduction in operation costs arerealized through the use of less expen-sive larger diameter electrodes, lessexpensive CO2 shielding gas, andreduced spatter. The use of a largerdiameter wire will reduce the actualweld time, and improve efficiency. Asignificant savings can also be realizedbecause the STT process operates withthe less expensive CO2 gas when usingsteel alloys. Reduced spatter translatesinto significant cost savings due to lesscleanup required of fixturing and theweld surface area prior to final surface

preparation, but also because more ofthe electrode stays in the weld joint,resulting in reduced electrode con-sumption. Finally, welding time isadded before cleaning accumulatedspatter from the gun nozzle.

Operating conditions are alsoimproved for welders who will be morecomfortable by the decrease in spatter,decrease in smoke levels, and reductionin arc radiation over Short Arc transfer.Creating a safer environment willincrease the operators comfort,concentration and confidence toproduce a weld with minimal variation.Because of its superior arc characteris-tics, welding with STT requires lessoperator skill in handling the torch,thus minimizing training.

While STT offers many benefits, itis also very important to understand itsdisadvantages and limitations com-pared to conventional short circuitingprocess. One of the first disadvantagesa manufacturer would notice is that theSTT power source is initially moreexpensive than a constant voltagepower source. This may be explainedby the use of a patent protectedtechnology and the cost savings thatare realized though its benefits. Thedeposition rates are lower than globu-lar, spray arc and pulse spray, but areequal to that of short circuit welding.As in pulsed spray welding, setting thewelding parameters for STT are quitedifferent than settings normally usedand may require additional training.Finally, the STT process differs fromthe conventional short circuitingprocess through its inability to performaluminum welding at this time.

Case StudiesThe release of a new technology

always promises remarkable advan-tages and benefits over an existing one.These promises carry no merit until thetechnology is implemented and itsbenefits validated by commercial end-users. The following case studies verifythe benefits being realized by manufac-turers, and the implications this newtechnology will have on the weldingindustry. These case studies are a crosssectional representation of the weldingindustry, with feature companies

specializing in stainless steel, structuralsteel, bridge steel, and light gauge steelapplications.

Advanced Bus Industries, L.L.C.(ABI) manufactures advanced designcustom vehicles. These bus-typevehicles are outfitted with leatherinterior, televisions, VCRs and evenGlobal Positioning Systems. Thecompany was challenged with increas-ing its productivity and adopting a newcommercial model. In changing overits Columbus, Ohio shop to accommo-date the new model, ABI revamped itsentire manufacturing process andturned to a new material for vehicleproduction stainless steel. But whenit comes to production, stainless steelpresents a welding challenge, as it is apoor conductor of heat and thereforeretains heat in the weld zone leading towarpage and distortion. To combat thisstainless steel welding problem, ABIturned to STT. With STT, cleanup timeat ABI has been dramatically reducedby more than 75 percent (Dodson,2000). Additionally, since operatorshave more control over the arc and theentire welding process, travel speedsare up. Both of these factors contrib-uted to increased vehicle production.Spatter is almost non-existent with theSTT, especially in critical areas whereseveral joints meet. states Ron Estes,the Weld Shop Supervisor for ABI.STT also provides more control of theweld puddle by offering additionalsettings for peak, background, tail-outand hot start, so there are many optionsto tailor the weld machine. Thebenefits of the STT process includereduced spatter, smoke and distortion all critical elements for high-qualityvehicle production. The reduced clean-up time offered by the STT represents asubstantial cost savings in labor to ABI.

J.N. Rowen Limited, one of theUnited Kingdoms leading independentstructural steel working companies, hasrecently branched into the design andfabrication of tubular structuresrequiring high integrity welds. Accord-ing to Lincoln (2000), after securingthe prestigious steel work contract forthe new Wimbledon No. 1 Court, J.N.Rowen employed the unique inverterbased STT process. The STT process


Table 1 Comparison of GMAW to STT.

Gas Metal Arc Welding Surface Tension Transfer

Metal Transfer Process Short Circuiting Transfer Modified Short Arc with the Amperageand Voltage changed based upon the

needs of the Arc

Voltages 16V To 22V 16V To 22V

Amperages Low Amperages: (30A to 200A) Two Amperage Levels: Peak Current (0A to 450A) Background Current (0A to 125A)

Wire Electrode Size Typically Smaller Diameters Typically Larger Diameters(0.025 in to 0.045 in) (0.035 and 0.045)(0.60 mm to 1.10 mm)

Shielding Gases: 100% CO2 (Lowest Cost) 100% CO2 (Lowest Cost) 75% Ar/25% CO2 Gas Mix Custom blended to meet the optimum

arc physics

Advantages All Position Welding Low Heat Input Low Cost Controlled Heat Input

All Position Welding Handles Poor Fit Up Minimal Spatter Can Use a Larger Wire Size Minimal Smoke Low Cost Gas Good Fusion

Limitations Spatter More Expensive Equipment Potential Lack of Fusion Limited to a Modified Short-Circuit Limited to Thin Material Mode

Costs $3,000 $6,000

Training/Skill Similar Similar

Materials Carbon and Low Alloy Steels Carbon and Low Alloy Steels Galvanized/Zinc Coated Galvanized/Zinc Coated Stainless and Nickel Alloys (plating unaffected on backside) Silicon Bronze and Copper Alloys Stainless and Nickel Alloys (with greatly

reduced spatter) Silicon Bronze and Copper Alloys

Industries Automotive Automotive Food and chemical processing Pipe and Pressure Vessel Consumer products Power Generation

Food and chemical processing Thin gauge consumer products


was seen as the next step in theircontinuing development of weldingtechnology; offering enhanced productquality, greater productivity andincreased profitability. The followingbenefits were identified throughout thisconstruction project:

Ease of Use - Optimum arccharacteristics were maintainedeven with variations in electrodeextension. This released thewelder from the need to maintainexact lengths and welding gunangles in order to produce asmooth, low spatter, high integ-rity weld. Full welder trainingand welder qualifications werecompleted within one day.

Controlled Arc Energy - Theplasma boost caused the arc tobroaden; melting a wide surfacearea, eliminating cold lappingand promoting good fusion, evenon heavier gauge materials.

Reduction in consumable costs -100% CO2 Shielding Gasproduced a gas savings in excessof 25%.

Increase in productivity -Operator friendly processreduced down time and operatorfatigue. High travel speeds forroot passes and all positionwelding substantially reducedoverall weld times. Also, thereduction in spatter translatedinto savings by minimizing oreliminating the labor time toremove spatter, plus savings intime from the ability to producehigh integrity welds in anyposition. This eliminated theneed to rotate and position thestructures for welding. Withover one thousand butts to weld,substantial savings on productiontimes were achieved.

XKT Engineering Inc is located onMare Island in Vallejo, California. Thecompanys location on the bay andaccess to barge transportation, make itthe perfect firm to handle bridgereinforcement and new constructionprojects. These projects require the useof pipe pilings that run anywhere from60 to 160 feet in length. To handle this

AWS code-quality work quickly andefficiently, XKT has turned to the STTprocess. According to Goetz (2000),this process is able to put in the pipesroot weld pass three to four times fasterthan the former process of stick weld-ing, in addition to being easier toperform for XKTs 23 certified welders.Prior to the STT, we were using a stickprocess with back-up bars, says CorkeyBates, Welding Engineer/ProductionManager for XKT Engineering, Inc.When welding a 24 diameter pipe, wewould use approximately 10 to 12consumable rods, which means a lot ofstarting and stopping. With the STT, weweld of the pipe at a time, so we onlystart and stop four times while layingthe root pass. This means increasedspeed in welding for the root pass. And,because the STT root pass cross-section(or nugget) is larger than in the past, thehot (or second) pass goes much faster aswell. Also, with fewer starts and stops,we have decreased the potential forwelding imperfections like porosity andcratering. Our welds must pass eitherradiographic or ultrasonic inspection toAWS D1.1 standards for cyclicallyloaded tension stress welds. With theSTT, we were easily achieving thesequality welds on our root pass, notesBates. We also noticed that the STTgenerated less spatter and fume than ourprevious stick process, which meant lessclean-up and the ability to go right tosubmerged arc for the fill and cappasses.

Honda employs about 850 associ-ates at its Marysville Ohio MotorcyclePlant. The plant produces about150,000 units a year, of which over90,000 are equipped with fuel tanksusing the STT weld equipment. Thetanks are fabricated from highlyformed sections of 22GA sheet steeland are then welded together with a100% penetration butt weld. Becauseof the thickness of the parts and thevarying contour of the seam, welding isoften a challenge. After changing toSTT, almost all of the defects Hondahad previously encountered with theconventional MIG process had beeneliminated and rejects were reduced bymore than 90% (Wall, 2000).

Summary and ImplicationsIndustrial Technology educators

make a significant contribution to thesuccess of manufacturers everydaywhen they teach students about thelatest technology in industry. The fieldof welding should be no exception andIT educators should integrate adaptivewaveforms such as STT weldingtechnology into their courses andlaboratory experiences. STT offersmany advantages primarily over theShort Arc means of metal transfer. Thegreatest advantages are the improvedweld qualities, reduced operation costsand improved operating conditions.The backbone of STT is its revolution-ary waveform technology whichcontrols current precisely and indepen-dently of wire feed speed during theentire welding cycle. This precisedigital control of the weld currentrepresents the next generation inwelding technology and a whole newfuture in industrial applications.

The implications for this emergingtechnology are far reaching as theautomotive, petroleum, and structuralsteel industries have just begun imple-mentation. The advent of STT hascaused manufacturers to grapple with aparadigm shift in their traditionalapplication of short-arc welding.These companies are now realizingsignificant cost savings by replacingtheir traditional MIG, stick welding,and fastener processes with STT. It isthis authors estimation that up to 75%of the current short-arc sheet metal andpipe welding applications could bejustifiably replaced by STT due to itstangible benefits of improved quality,lower costs, and improved operatingconditions. Within the next 10 years, itis possible that STT will become thepredominant steel alloy weldingprocess used in U.S. manufacturing;therefore it is critical that we beginpreparing our future technologists andengineers to exploit and optimize thislatest high-tech process.


ReferencesDeRuntz, B. D. (2001). Surface Tension

Transfer welding in manufacturing.Selected Paper presented at theNational Association of IndustrialTechnology Conference, DetroitMichigan, 20-26.

Dodson, D. (1999). New weldingtechnology eases FGD wallpaper-ing. Power Engineering, 103(6), 38-42.

Dodson, D. (2000). New weldingequipment puts stainless steelvehicle production in the fast lane.Retrieved June 1, 2001, from http://www.lincolnelectric.com/products/tech/abi1.asp

Goetz, J. (2000). New equipmentspeeds up the welding of pipepilings for bridge building andreinforcing., 2001, from http://www.lincolnelectric.com/products/tech/gemco.asp

Lincoln, U. K. (2000). STT case study -J.N. Rowen Ltd. at Wimbledon.Retrieved June 1, 2001, from http://www.lincolnelectric.com/products/tech/casestt.asp

Stava, E. K. (2000). The surfacetension transfer power source, Anew, low-spatter arc weldingmachine. Retrieved June 1, 2001,from http://www.lincolnelectric.com/products/tech/low_spatter.asp

Stava, E. K. (2001). Waveform controlspeeds root pass. Welding Design &Fabrication, 74(2), 39.

Wall, K. (2000). Honda improvesproductivity with STT weldingprocess. Retrieved June 1, 2001,from http://www.lincolnelectric.com/products/tech/honda1.asp

Assessing the Benefits of Surface Tension Transfer®

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